Heat treatment of molten carbonaceous material prior to its conversion to carbon fibers and other shapes

ABSTRACT

A METHOD IS PROVIDED FOR THE PRODUCTION OF CARBON SHAPED ARTICLES IN THE FORM OF FILAMENTS, YARNS, RIBBONS, FILM, SHEETS, TUBES AND THE LIKE. THE SHAPED ARTICLES RESULTING FROM THE PROCESS ARE LIKEWISE PROVIDED. THE METHOD COMPRISES THE STEPS OF A. HEATING A NATURAL OR SYNTHETIC ORGANIC SUBSTANCE, SUCH AS POLYVINYL CHLORIDE, POLYACRYLONITRILE, PETROLEUM PITCH, COAL PITCH AND OTHERS, IN THE PRESENCE OF AN INERT GAS TO A TEMPERATURE IN THE RANGE OF ABOUT 300* TO 500*C., WHICH TEMPERATURE IS BELOW THE CARBONIZATION TEMPERATURE OF SUCH SUBSTANCE, B. BRINGING THE TEMPERATURE OF THE SUBSTANCE TO A TEMPERATURE BELOW THE CARBONIZATION TEMPERATURE IN AN INERT ATMOSPHERE AND FORMING THE SUBSTANCE INTO A DESIRED SHAPE, C. CONTACTING THE SHAPED ARTICLE WITH AN OXIDIZING GAS, AND D. SUBJECTING THE ARTICLE TO CARBONIZATION BY HEATING IN AN INERT ATMOSPHERE AT A TEMPERATURE SUBSTANTIALLY ABOVE 500*C. THE SHAPED ARTICLES ARE USEFUL FOR A WIDE RANGE OF USES, AS FOR EXAMPLE, THERMAL INSULATION MATERIALS, CARBON ELECTRODES, AND THE LIKES.

United States Patent 1 Otani 1 Feb. 13, 1973 [54] HEAT TREATMENT OFMOLTEN CARBONACEOUS MATERIAL PRIOR TO ITS CONVERSION TO CARBON FIBERSAND OTHER SHAPES [75] lnventor:

[73] Assignee:

Sugio Otani, Hishi-machi, Japan Kureha Kagaku Kogyo Kabushiki Kaisha,Tokyo-to, Japan 221 Filed: July7, 1970 [21] Appl.No.:52,8l7

[30] Foreign Application Priority Data Nov. 1, 1963 Sept. 1, 1964 Japan..38/58942 Japan ..39/49772 [52] U.S. Cl. ..264/29, 23/2092, 264/83,264/85, 264/176, 264/D1G. 19 [51] Int. Cl. .C0lb 31/07, D06m 11/12, D06m11/04 OTHER PUBLICATIONS S. Otani, et aL, On The Raw Materials of MPCarbon Fiber, 1966, Vol. 4, Carbon, Pergamon Press Ltd Great Britain,425-432 Primary Examiner-Donald .1. Arnold Assistant Examiner.1ohn H.Miller Attorney-Wenderoth, Lind & Ponack ABSTRACT A method is providedfor the production of carbon shaped articles in the form of filaments,yarns, ribbons, films, sheets, tubes and the like. The shaped articlesresulting from the process are likewise provided. The method comprisesthe steps of a. heating a natural or synthetic organic substance, suchas polyvinyl chloride, polyacrylonitrile, petroleum pitch, coal pitchand others, in the presence of an inert gas to a temperature in therange of about 300 to 500 C., which temperature is below thecarbonization temperature of such substance,

b. bringing the temperature of the substance to a temperature below thecarbonization temperature in an inert atmosphere and forming thesubstance into a desired shape, I

c. contacting the shaped article with an oxidizing gas, and

d. subjecting the article to carbonization by heating in an inertatmosphere at a temperature substantially above 500 C.

The shaped articles are useful for a wide range of uses, as for example,thermal insulation materials, carbon electrodes, and the like.

8 Claims, No Drawings HEAT TREATMENT OF MOLTEN CARBONACEOUS MATERIALPRIOR TO ITS CONVERSION TO CARBON FIBERS AND OTHER SHAPES This inventionrelates to techniques in the production of carbon structures includingshaped articles and more particularly to a new method for producingshaped articles of carbon from molten baked organic substances.

While the present invention is intended to cover the production ofcarbon structures including shaped articles in the form of filaments,yarns, ribbons, films, sheets, tubes, and the like, the followingdisclosure will be presented principally with respect to filaments as arepresentative structure for the sake of brevity, other forms beingreadily produced according to the invention by apparatus and processesgenerally known in the art, the detailed operating conditions beingsuitably modified.

Heretofore, as a method of producing carbon filaments, the method ofcarbonizing filaments of organic substances such as cellulose filamentsand acrylonitrile filaments while the filaments are retained in theiroriginal form has been known. This method requires starting materials ofhigh cost and is accompanied by certain difficulties such as thesusceptibility of the filaments to loss of their original form at theheat-treatment process and insufficient mechanical strength of thecarbon filaments so produced.

It is a general object of the present invention to provide a method forproducing carbon structures whereby the above described difficulties areovercome, and, moreover, to provide carbon structures havingunprecedentedly high strength, high uniformity of cross section, andhigh lustre, in spite of the use of low-cost starting materials.

It is another object to include in the method steps an oxidationtreatment, particularly an ozone treatment, in certain instances toimprove the properties of the resulting product.

According to the present invention there is provided a method forproducing shaped articles of carbon which comprises melt extrusion andstretching of a molten stock resulting from the baking, at a temperatureof from 300 to 500 C., of natural or synthetic organic substances. Themelt extrusion is accomplished at the baking temperature of saidstarting material or in the temperature region therebelow. The extrusionis followed by an oxidation treatment and a carbonization treatment ofthe resulting filament under nitrogen.

The present invention is based on the remarkable discovery that whenmany kinds of organic compounds are heated at a suitable temperature inthe range from 300 to 500 C., the resulting molten products shortlybefore carbonization exhibit excellent spinnability.

Examples of natural or synthetic organic substances which can be used inthe practice of the present invention are synthetic high-polymersubstances such as polyvinyl chloride and polyacrylonitrile, andhighpolymer or low-polymer organic substances such as petroleum pitch,coal and coal pitch, distillation residues of benzyl chloride andchlorobenzene, and byproducts of DDT.

The unique phenomenon is observed in the following manner using the caseof polyvinyl chloride by way of example. When the heating temperature ofpolyvinyl chloride is gradually raised in an inert gas, colorationthereof begins at 160 C., and at a temperature between 200 C. and 250 C.the entire substance expands and darkens in color, eventually becomingblack. Up to this temperature the baked substance is soft, but whenheated further to a temperature between 250 C. and 300 C. it contractsin volume and becomes hard. At this stage there takes place thedehydrochlorination (removal of hydrogen chloride) which may beconsidered to be the main reaction of the thermal decomposition ofpolyvinyl chloride.

Then, if the raising of the temperature is continued, cyclization andthree-dimensional crosslinking will occur, and at a temperature between400 C. and 420 C., the three-dimensional linking will be abruptly cutoff. The resulting substance in the pitch state is considered to be apolycyclic aromatic compound closely related in structure to aromaticplane component making up amorphous carbon. If the temperature is raisedto 500 C., this polycyclic aromatic compound will again harden withoutremarkable weight loss and become a lustrous, massive substance which isinsoluble and unmeltable.

If the aforementioned substance in the pitch state is cooled to C., orlower temperature, it will also become a black, lustrous solid. Thissolid can be readily dissolved in benzene, chloroform, and othersolvents and melts when heated over 150 C.

Examination of X-ray and absorption spectrum conducted at each of theabove described stages of procedure, shows to be that when thetemperature of the baked substance reaches 500 C., the substance becomescarbon in terms of crystallography.

The term carbon is herein used in the crystallographical meaning todesignate that structure wherein condensed polycyclic planes arearranged in laminar form. The temperature region higher than so calledthermal-decomposition temperature and lower than the carbonizationtemperature is herein called the temperature region shortly before thecarbonization. This temperature region differs with different substances and, even with the same substance, it varies somewhat dependingon the surrounding atmosphere and the rate of heating. However, in anycase this temperature region does not depart greatly from the range ofapproximately from 300 to 500 C.

The natural or synthetic organic substance used in the method of thisinvention is heated to a temperature in the region of 300 to 500 C.shortly before the carbonization in a nitrogen gas (N carbon dioxide gas(CO or some other inert gas atmosphere, or under conditions whereinoxygen does not exist. At this stage, it appears that both high and lowmolecular weight substances, while the material being treated, undergofission or coupling to molecules composed principally of polycyclicaromatic structure, whereby both types of substances change into moltenbaked substances having substantially the same range of molecular sizeand having plasticity.

Moreover, by melt spinning or otherwise melt extruding the substance inthis state by a suitable method, the substance is shaped into thedesired form such as filaments, which, as a result of the succeedingoxidation treatment with air or other agent and heat treatment, is

unexpectedly rendered unmeltable in a very easy manner. By subjectingthis extruded material further to carbonization or graphitization, it ispossible to obtain the objective carbon or graphite article such asfilaments.

When this temperature is 300 C. or less, sufficient fission andrecoupling of the carbon-carbon chain do not occur. When the temperatureexceeds 500 C., recoupling of the carbon-carbon chain occurs, and theplasticity of the substance is lost (the temperature producing thisstate being called the carbonization temperature), and accordingly thefilaments or other structure which is the objective product of theinvention cannot be obtained.

That is, by heating the starting material at a suitable temperaturebelow the carbonization temperature within the range of from 300 to 500C. for a suitable time, for example, from 5 minutes to hours, thenecessary rearrangement of the molecules is effected, and a plasticmaterial suitable for melt extrusion (for example, melt spinning) isobtained.

The baked substance in a pitch state, heated for a suitabletime at asuitable temperature in the range of from 300 to 500 C. may be cooledonce, preserved, and then reheated for the succeeding melt extrusionprocess, or it may be immediately subjected to an appropriatetemperature change in order to adjust its molten viscosity for thesucceeding extrusion process and then extruded (for example, spun asfilaments).

A temperature below the aforementioned baking temperature is ordinarilyselected for the extrusion temperature. The extrusion process ispreferably carried out in an atmosphere of an inert gas as mentionedhereinabove, but the extruded article (for example, filaments) is causedto contact air or an oxidizing gas at the extrusion temperature or lowertemperature for a number of minutes or longer. it has been found thatthis process is remarkably effective. it appears that, by this process,recoupling occurs together with the alinement of the molecules in theextrusion (filament) direction during the succeeding stretching processor subsequent process to which the extruded article is subjected,whereby high polymer carbon material (carbon filaments) of insoluble andunmeltable characteristic is further produced.

The carbon extruded article (filaments) obtained in the above manner aresubsequently carbonized amply in a state wherein they are not in contactwith oxidizing gases such as air. The rate of heating during this stepis preferably 10 C./minute or lower. When this tempera ture rises above600 to 700 C. to approximately l,500 C., the shaped article (filaments)assumes mechanical strength of practical magnitude. lf necessary, thearticle can be further subjected to heat treatment at 2,000 C. or highertemperature to produce a graphite article (filaments).

Carbon filaments or graphite filaments produced in the above describedmanner can be used effectively for a wide range of uses, examples ofwhich are thermal insulation materials, carbon electrode, other basicmaterials for carbon and graphite products, reinforcement materials ingeneral, reinforcement materials for various products made of syntheticresins, electroconductive and heating mats, heat-resistant packings,fillers for electroconductive paints, and various resistance materialsfor electronics.

In order to indicate more fully the nature of the invention a fewexamples of typical procedure are presented hereinbelow.

EXAMPLE 1 grams of polyvinyl chloride powder was heated in nitrogen gas(inert gas) at a rate of 1 C./minute up to 400: 5 C., which temperaturewas maintained'for one hour, whereupon 30 grams of a molten bakedsubstance was obtained. (When this substance is cooled to roomtemperature, it solidifies, and, further, when it is crushed, it becomesa brownish black powder.)

This substance was heated rapidly to 275:5 C. and melted as carbondioxide gas was caused to flow over its surface. The molten substancewas then extruded into air and stretched to produce filaments ofdiameters of from 20 to 30 microns. These filaments were heated fromroom temperature at a rate of 5 C./minute up to 250 C. at whichtemperature N gas was caused to flow to replace air, and the heating wascontinued at the same rate up to 900 C., which was then maintainedconstant for ten minutes. The filaments were then left to coolnaturally.

As a result, carbon filaments having a strength (tenacity) of 4 X 10grams/cm. were obtained.

In contrast, when polyvinyl chloride filaments were heated gradually inair to 300 C. and then baked in N gas up to 500 C., the strength of theresulting carbon filaments was only 9 X 10 grams/emf.

EXAMPLE 2 The distillation residue which is a by-product in theproduction of benzyl chloride from the reaction of chlorine with tolueneis a brownish black, tar-form substance. 250 grams of this substance washeated to 400 C. in a stream of N gas and subjected to dry distillation, the same temperature then being maintained for a further 30minutes to remove low-temperature boiling point components anddecomposition products.

As a result of this dry distillation treatment, 100 grams of apitch-form product was obtained. This resulting substance is in theliquid state at 400 C., and at room temperature it is a brownish black,lustrous solid, which when crushed becomes brown in color. Thissubstance was rapidly heated and in a molten state at from 220 to 240 C.was spun into air, whereupon it was readily shaped into filament form.

These filaments so spun were heat-treated in air for 30 minutes at C.and for 30 minutes at 200C.,and then baked in N, gas in the same manneras set forth in Example 1.

As a result, carbon filaments exhibiting a strength of 4.2 10 grams/cm.were obtained.

EXAMPLE 3 250 grams of the same distillation residue of benzyl chlorideas used in Example 2 was heated to 400 C. in a stream of N gas and drydistilled. The temperature was maintained for a further 30 minutes, andthen, as the pitch state of the resulting substance was maintained, itwas gradually cooled from 220.to 240 C. and then spun at thistemperature, whereupon a filament-form product was readily obtained. Bysubjecting this product to the same heat treatment as set forth inExample 2, excellent carbon filaments were obtained.

The present invention, in another important aspect thereof, provides anew oxidation treatment step for the product subsequent to its meltextrusion. By this treatment great saving is effected in the time andheat supply necessary for the process of producing carbon filaments andother carbon structures. This treatment affords not only economicadvantage but also substantial improvement in the quality of theproduct, particularly making possible the production of carbon filamentsof remarkably high strength.

When carbon filaments are produced from molten baked substances, theoxidation treatment subsequent to baking is particularly an importantproblem. Heretofore, it has been considered that the only possiblemethod for effecting this treatment is that of heating and oxidizing thefilaments at a low temperature in air. According to the present it hasbeen discovered that by carrying out oxidation due to ozone in atemperature range below 100 C., the beneficial effects of the treatmentas mentioned above can be attained in a very effective manner.

While it is desirable that the melt extrusion process be carried out inan atmosphere of inert gas such as N Ar, and CO as describedhereinabove, the extruded filaments, according to the method of thisinvention, are caused to contact air or an oxidizing gas at theextrusion temperature or lower temperature for several minutes orlonger.

In one embodiment of this process according to the invention, atreatment is carried out on the filaments with air containing ozone orwith oxygen gas for a suitable time of 7 hours or less at a suitabletemperature in the range of from room temperature to 100 C., andimmediately thereafter the filaments are subjected to oxidationtreatment in air at a temperature range up to 260 C.

Although the chemical change occurring because of the ozone treatment isnot fully apparent, it appears, from the increase in weight and resultof measurement of infrared absorption spectrum, that an addition ofoxygen is effected, and this, in itself, forms a three-dimensionalbridge structure while, at the same time, effectively promoting theformation of bridge couplings due to the succeeding oxidation in air.

The filaments so obtained are then subjected to ample carbonizationtreatment in a state wherein they are not in contact with an oxidizinggas such as air as described hereinbefore.

Measurement of the relationship between the characteristics of thecarbon filaments obtained in the above described manner and the ozonetreatment process constituting a unique feature of this inventionreveals a remarkable increase in the strength of the carbon filaments asis indicated in the examples set forth hereinafter. This ozone treatmentnot only is highly effective in improving the properties of the carbonfilaments obtained but also greatly contributes to the economicalcarrying out of the preceding treatment process.

That is, as shown in Example 4 to follow, by carrying out a 3-hour ozonetreatment at 70 C., it is possible to shorten substantially and freelythe time of preparatory oxidation treatment necessary for rendering theproduct insoluble without difficulty. This desirable effect cannot beexpected from a simple heating in air.

Moreover, a treatment imparting such a great effect on the quality ofcarbon filaments has heretofore been unknown as far as we are aware.

This effect is particularly pronounced in the case where the diametersof the filaments are small. The reason for this appears to be that inthe case of small diameters, the surface area increases, whereby thesurface energy increases, and, at the same time, the ozone treatment iscaused to take effect with increased thoroughness throughout the entirefilament structure.

EXAMPLE 4 grams of polyvinyl chloride powder was heated in nitrogen gas(inert gas) at a rate of 1 C./minute up to 400 i 5 C., which temperaturewas maintained for 1 hour, whereupon 30 grams of a molten bakedsubstance was obtained as described in Example 1.

This substance was heated rapidly to 275 i 5 C. and melted as N gas wascaused to flow over its surface. The molten substance was then extrudedthrough a jet (spinneret) into air to produce filaments having diametersof from 5 to 30 microns. These filaments were ozone treated under thefollowing conditions.

1. Treatment temperatures: 25, 50, and 70 C.

2. Ozone concentration: 10.4 grams/cubic meter;

ozone flowrate: liters/hour.

3. Treatment time: 3 hours in all cases.

It was found that this treatment caused weight increases of from 0.5 to1.2 percent in the filaments, and it was verified through infraredabsorption spectrum observation that a C=O radical in the vicinity of1,700 cm.- was formed. It was found further that, in the case oftreatment at 70 C., a part of the product was rendered insolublerelative to benzene.

The filaments so treated were then heated in air to 260 C., andmaintained at this temperature for 1 hour,

after which they were carbonized by the normal procedure by heating in Ngas to l,000 C.

The relationship between the strength of the resulting carbon filamentsand the ozone treatment temperature is as indicated in the followingTable 1.

TABLE 1 Ozone treatment Strength (grams/cm?) None 4.5 X 10 at 25 C. 5.5x 10 at 50 C. 6.5 x 10 at 70C. 7.3 x 10 NOTE: Diameter of samplefilaments was 15 microns.

EXAMPLE 5 Samples similar to those in Example 4 except that the spinningconditions were varied suitably to obtain filaments of differentdiameters were prepared, which were all ozone treated at 70 C. for 3hours. The other treatment conditions were the same as those inExample 1. As a result, carbon filaments having final diameters of l5,l0, and 8.5 microns were obtained.

The results of measurement of the strengths of the filaments so obtainedare as shown in the following TAble 2, from which it is apparent thatthe effect of the ozone treatment is pronounced in the case of filamentsof small diameters, and that filaments of very high strength can beobtained in this case.

TABLE 2 Filament diameter (microns) Strength (grams/cm!) l5 7.3 X l013.5 X 10 8.5 l6.0 X 10 It should be understood, of course, that theforegoing disclosure relates to only particular examples of theinvention and that it is intended to cover all changes and modificationsof the examples of the invention herein chosen for the purposes of thedisclosure, which do not constitute departures from the spirit and scopeof the invention as set forth in the appended claims.

What I claim is:

l. A method for the production of carbon shaped articles whichcomprises:

a. heating an organic substance selected from the group of natural andsynthetic substances which 1. undergo fission or coupling to moleculescomposed principally of polycyclic aromatic structure, and (2) are in amolten state at a temperature of about 300 to 500 C. in an inertatmosphere in the presence of an inert gas at a temperature in the rangeof about 300 C. to about 500 C. which temperature is below thecarbonization temperature of the substance for a time sufficient tocause the material to undergo fission or coupling to molecules composedprincipally of polycyclic aromatic structure whereby both types ofsubstances change into molten baked substances having substantially thesame range of molecular size and having plasticity,

b. bringing the thus-heated organic substance to a forming temperaturewhich is below the range of 300 to 500 C., and higher than the softeningpoint thereof in an inert atmosphere, and forming the mass into thedesired shape,

c. oxidizing the resultant shaped article by contacting the same with anoxidizing gas at a temperature between room temperature and the shapingtemperature, and

d. subjecting the resultant article to carbonization by heating sucharticle in an inert atmosphere to a temperature substantially above 500C.

2. A method for the production of carbon shaped articles whichcomprises:

a. heating an organic substance selected from the group of natural andsynthetic substances which 1. undergo fission or coupling to moleculescomposed principally of polycyclic aromatic structure, and

. are in a molten state at a temperature of about 300 to 500 C. in aninert atmosphere in the presence of an inert gas at a temperature in therange of about 300 C. to about 500 C. which temperature is below thecarbonization temperature of the substance for a time sufficient tocause the material to undergo fission or coupling to molecules composedprincipally of polycyclic aromatic structure whereby both types ofsubstances change into molten baked substances having substantially thesame range of molecular size and having plasticity, b. subsequently coomg the hus-heated organic substance to room temperature, reheating to aforming temperature which is below the range of 300 to 500 C., andhigher than the softening point thereof, in an inert atmosphere, andforming the mass into the desired shape,

. oxidizing the resultant shaped article by contacting the same with anoxidizing gas at a temperature betweenroom temperature and the shapingtemperature, and

. subjecting the resultant article to carbonization by heating sucharticle in an inert atmosphere to a temperature substantially above 500C.

3. A method as in claim 2 wherein the organic substance is a memberselected from the group consisting of polyvinyl chloride,polyacrylonitrile, petroleum pitch, coal, coal pitch, distillationresidue of benzyl chloride and chlorobenzene, and by-products of DDT.

4. A method as in claim 2 wherein the heated organic substance resultingfrom (a) is melt extruded to form a fiber.

5. A method as in claim 2 wherein the heated organic substance resultingfrom (a) is melt extruded to form a film.

6. A method as in claim 2 wherein the oxidizing gas in (c) is selectedfrom the group consisting of air, ox-

ygen gas and air containing ozone.

7. A method as in claim 2 wherein the shaped article is contacted withair containing ozone at a temperature in the range of from roomtemperature to C.

8. A method as in claim 2 wherein the shaped article is contacted withair containing ozone at a temperature of from room temperature to 100C.for less than 7 hours and subsequently contacted with air at atemperature of up to 260 C.

